This invention relates to the measurement or control of fluid flow rate and, more particularly, to the application of digital techniques thereto.
Conventionally, fluid flow rate is measured by a flowmeter, such as a venturi meter, an orifice meter, or a turbine meter. In a venturi meter and an orifice meter, the flow rate is proportional to the pressure in a fluid passage having fixed cross-sectional dimensions. In a turbine meter, the flow rate is proportional to the angular velocity at which the turbine rotates. For any particular meter, flow rate is proportional to the measured parameter within a limited range of flow rates. Therefore, to make accurate measurements over a wide range of flow rates, a number of particular meters having different dimensions must often be employed, each covering a segment of the range.
In a conventional analog fluid flow control system, the flow rate is controlled by positioning a plug located in the fluid stream. The degree to which the plug impedes flow governs the flow rate. In order to establish a set point value of flow rate, a flowmeter generates a signal representative of the actual value of flow rate, which is compared with a command signal representative of the set point value, and the plug position is adjusted by a control loop until the actual value corresponds to the set point value. In large oil refineries, chemical plants, and other processing facilities, supervisory digital computers run the operations by issuing set point commands to the individual flow control systems and receiving data concerning the status of the operations. The limited range of present flowmeters mentioned in the preceding paragraph, however, restricts the range of set point values that an analog fluid flow control system can accurately accommodate. Further, it is difficult to derive the actual value of flow rate indirectly by calculation because flow rate depends in part on the effective cross-sectional area of the flow passage which is a complex function of the plug position.
In a digital fluid flow control system, a plurality of individually actuatable, value weighted digital bistable valve elements in parallel interconnect an upstream manifold to a downstream manifold. Each valve element exclusively assumes either an open state in which fluid flows from the upstream manifold through the valve element to the downstream manifold, or a closed state in which no fluid flows from the upstream manifold through the valve element to the downstream manifold. The effective cross-sectional orifice areas of the flow passages through the respective valve elements are weighted according to a binary code, e.g. a geometric progression of two, thereby value weighting the digital valve elements. The valve elements are actuated by binary signals weighted according to the same binary code as the respective valve elements to which they are coupled. The sum of the effective orifice areas of the valve elements in the open state is related to the binary number or value represented by the actuating signals in the binary code. Recent improvements in the design of digital fluid flow control systems have virtually eliminated any interaction between valve elements, i.e., any dependence of the effective orifice area of one valve element upon the states of the other valve elements, and minimized the effect of pressure variations and ambient conditions on effective orifice areas. Consequently, the sum of the effective orifice areas of the open valve elements can be made proportional to the binary number represented by the binary actuating signals to a high degree of accuracy.